Was it leaking before you checked the valves? if not then for sure you missed something. A leaking valve will generally not cause crankcase pressure, bad rings could. is your crankcase vent pinched? I've seen that happen. where is it leaking oil? there is an oring under one of the cam caps that if missing will cause a leak out the small hole in the side of the head, this is from the spark plug area. there is always some pressure as it warms up.